1. Field of the Invention
The present disclosure relates to an organic light emitting functional device with organic electron injection layer to improve the injection of electrons from the cathode in an organic light emitting diode. The present disclosure particularly relates to the use of commonly employed electron transport layer 4,7-di phenyl-1,10 phenanthroline (herein after called as BPhen) and another organic semiconductor Tetracyano quino dimethane (herein after called as TCNQ) and optimizing the thickness and doping percentage of the composition in an organic light emitting device.
2. Discussion of the Background Art
Balanced injection and transport of charge carriers from both the electrodes i.e. cathode as well as anode side is crucial for highly efficient OLEDs. The choice of these charge transport and charge injection layers is also an important issue. The large interface barrier offered by charge injection layers results in increased potential drop across these layers. Since OLEDs are current driven devices and large current passes through these devices, these potential drops results in energy loss and efficiency loss. Further large potential drop across these layers causes Joule heating resulting in device failure. Therefore, efficient organic materials based devices require reduction of barrier faced by the charge carriers while traversing their path from the respective electrodes to the active layer of the device. For this purpose, several attempts have been made either by using electron injecting buffer layers for the ease of injection of electrons from the cathode or by using low work function cathode.
Reference may be made to U.S. Pat. No. 4,885,211 wherein the cathode is comprised of a layer of a plurality of metals, other than alkali metals, like Ag, Al, Mn, Cu, Au, Sn, In etc.
Reference may be made to U.S. Pat. No. 5,776,622 wherein a bilayer electrode which comprises of a fluoride layer contacting the emissive layer and a conductive layer contacting the fluoride layer.
Reference may be made to U.S. Pat. No. 6,140,763 wherein doped cathode (Li doped Al) has been used as interfacial electron injection layer.
The use of low work function cathode can effectively improve the electron injection but such metals are too reactive and react readily with water and oxygen and thus affect stability of the device. Another method is to use an electron injection layer between the cathode and the electron transport layer which is usually composed of n-type doped electron transport materials.To facilitate the injection of electrons from the cathode side, n-type doping with alkali metals is also done (Zugang Liu, O. V. Salata, Nigel Male; “Improved electron injection in organic LED with lithium quinolate/aluminium cathode”; Synthetic Metals, Volume 128, Issue 2, 30 Apr. 2002, Pages 211-214).Reference may be made to U.S. Ser. No. 10/173,682 and, EP1648042 wherein the fabrication of organic light emitting diodes has been carried out by employing n-type doping with alkali metals to facilitate the injection of electrons from cathode side.
Reference may be made to U.S. Pat. No. 7,070,867B2 which discloses OLEDs having n-type doping.
Reference may be made to U.S. Pat. No. 7,507,649—wherein Cs has been doped into BPhen to enhance electron injection.
Reference may be made to patent US2007/0034864A1 wherein a buffer structure is disposed between the organic layer and the cathode. This buffer structure consists of two layers, a first layer containing an alkali halide over the electron transport layer and a second buffer layer containing a metal/metal alloy is provided over the first buffer layer.
Reference may be made to IPC8: AH01J162FI, USPC Class: 313504: Nov. 13, 2008 wherein a mixed electron-transporting layer including 4,7-diphenyl-1,10-phenanthroline known as Bphen and tris (8-quinolinolato) aluminum (III) (Alq) as co-host, with 2% Li metal has been employed and Yasuhisa Kishigami, Kenji Tsubaki, Yukihiro Kondo and Junji Kido; “White Organic Electroluminescent Devices Having a Metal-Doped Electron Injection Layer” (Super-Functionality Organic Devices IPAP Conf. Series 6 pp. 104-107), Proc. Int. Symp wherein White Organic Electroluminescent Devices having a Cs doped BPhen electron injection layer has been proposed.
Alq3, phenanthroline and other electron transporting materials have been successfully doped with Li to significantly enhance their conductivity. [(a) “Lithium doping of semi conducting organic charge transport materials.” Parthasarathy G., Shen C., Kahn A., Forrest S. R. J. Appl. Phys. (2001), 89(9), 4986 4992. (b) “Low-voltage inverted transparent vacuum deposited organic light-emitting diodes using electrical doping.” Zhou X., Pfeiffer M., Huang J. S., Blochwitz-Nimoth J., Qin D. S., Werner A., Drechsel, J., Maennig B., Leo K., Appl. Phys. Lett. (2002), 81(5), 922 924, (c) US patent 2006/0251922A1 wherein two electron injection layers have been used, where first electron injection layer is Alq+1.2 vol %. Li and second is HATCN (d) “Efficient multilayer organic light emitting diode” Liu Z., Pinto J., Soares J., Pereira E., Synthetic Metals (2001), 122(1), 177-179 (e) “Electron structure of tris (8-hydroxyquinoline) aluminum thin films in the pristine and reduced states.” Johansson N., Osada T., Stafstrom S., Salaneck W. R., Parente V., Dos Santos D. A., Crispin X., Bredas J. L., J. Chem. Phys. (1999), 111(5), 2157 2163. (f) “Bright organic electroluminescent devices having a metal-doped electron-injecting layer.” Kido Junji, Matsumoto, Toshio, Appl. Phys. Lett. (1998), 73(20), 2866 2868]. Reference may be made to patent US2006/0269782 wherein Bphen+1.2 vol % Li and Alq+1.2 vol % Li serve as an electron injection layer.
Reference may be made to patent US2003/0230980 wherein the n-doped layer comprises BPhen:Li (1:1) ratio, United States Patent 2008/0227357 wherein LiF is used an electron injection layer to balance the charge carrier injection from cathode and anode respectively and Lian Duan, Qian Liu, Yang Li, Yudi Gao, Guohui Zhang, Liduo Wang, Yong Qiu (“Thermally decomposable Lithium Nitride as an electron injection material for highly efficient and stable OLEDs”, J. Phys. Chem. C 2009, 113, 13386-13390).
Reference may be made to U.S. Pat. No. 5,677,572 wherein a non-conducting layer such as LiF and MgO has been used between Alq and Al to improve the electron injection. US patent 2008/0093981A1 which proposes a buffer electron injection layer between cathode and emissive layer that is a composition containing molar ratio of benzoxazole derivative (electron acceptor) to an electron donating compound (for example TTF), Szu-Yi Chen, Ta Ya Chu, Jenn Fang Chen, Chien Ying Su (“Stable inverted bottom-emitting organic electroluminescent devices with molecular doping and morphology improvement” Applied Physics Letters 89, 053518, 2006), Xiang Zhou, Martin Pfeiffer, Jing S. Huang, Jan Blochwitz, Ansgar Werner, and Karl Leo (“Inverted OLEDs with Electrically Doped Carrier Injection and Transport Layers” Mat. Res. Soc. Symp. Proc. Vol. 725, 2002 Materials Research Society) where Li doped BPhen is used as an ETL and U.S. Pat. No. 7,501,755B2 wherein electron injection layer is represented by the formula AxByOz where A is an alkali metal/alkaline earth metal and B is a group VIII metal (0<x<=2, 0<y<=3, 0<z<=6, for example LiNiO2, LiCoO2).
The drawbacks of these attempts are that the metals in organic semiconductors interfere with luminescence properties. Metal atoms being highly mobile readily diffuse through organic materials leading to degradation due to formation of trapping centres and quenching sites. Also particularly, there are two potential drawbacks of doping with Li. The first drawback of Li doping is that the number of free carriers generated by Li doping is far less than the amount of Li that is doped into the film (carriers/Li<10%). The low yield of free carriers is thought to be due to the formation of charge transfer complexes, or tightly bound ion pairs [“Investigation of the interface formation between calcium and tris-(8-hydroxy quinoline) aluminum.” Choong V.-E.; Mason, M. G.; Tang, C. W.; Gao Yongli; Appl. Phys. Lett. (1998), 72(21), 2689-2691]. A second problem of Li doping is that Li may be highly mobile, readily diffusing throughout a device. Li diffusion into layers that are not meant to be redox doped leads to marked degradation of device performance due to the formation of trapping or quenching sites. Clearly the problems with Li doping are related to its high charge density and small size. Also Lithium is sensitive to oxidation. Apart from this, these attempts also require the handling of reactive metals. These metals are chemically reactive and are susceptible to atmospheric oxidation and corrosion also. Fabrication of the organic based device can only be carried out at considerably high temperatures in case alkali metals are used as n-type dopants. Furthermore, it requires a high doping ratio which in turn alters the matrix properties. Furthermore in the case of LiF, due to its high electrical insulation, its deposition as an electron injection layer in OLEDs has a thickness restriction (less than 1 nm), which is difficult to control.
Not withstanding these developments, there remains a need for efficient and stable electron injecting material to be used as a buffer layer between the cathode and the electron transport layer. 4,7-di Phenyl-1,10 Phenanthroline (BPhen) has been widely used in organic semiconductors based devices as an electron transporting as well as hole blocking layer. (US Patent No 20050074629, U.S. Pat. 7,179,543), (“Efficient and extremely long-lived organic light-emitting diodes based on dinaphthylperylene”; Viktor V. Jarikov, Denis Y. Kondakov, Christopher T. Brown; J. Appl. Phys. 102, 104908 (2007)), (“Efficient white organic light-emitting devices using 4,7-diphenyl-1,10-phenanthroline as block layer; Huishan Yang, Yanwei Shi, Yi Zhao, Jingying Hou and Shiyong Liu; Journal of Luminescence, vol 128, Issue 2, Pages 367-370) and many others. In addition to the above, extra thickness of insulating LiF which blocks electron injection can cause much problem when LiF is used in electron transport layer. The main object of the present disclosure is to provide an organic light emitting functional device with organic electron injection layer.
Another objective of the present disclosure is to provide an electron injection layer composed of most commonly used electron transport material 4,7 di phenyl 1,10 phenanthroline (BPhen) doped with another organic semiconductor Tetracyano quino dimethane (TCNQ).
Yet another object is to provide an electron injecting layer which is composed of only organic based materials and thus does not require metallic dopants.
Still another object of disclosure is to provide an electron injection layer which does not require high deposition temperature compared to that of alkali metals doping and requires a considerably low and practical amount of doping. Yet another object of disclosure is that the electron injection layer can be deposited uniformly with a thickness of 15 Å or 20 Å.
Another object of the present disclosure is to fabricate highly efficient multilayered organic light emitting diodes using this electron injection layer which will be useful in display applications as well as general lighting applications.
Another object of the present disclosure is to fabricate stable organic light emitting functional device with organic electron injection layer compared to usually employed n-type dopants containing LED.